Within the printing industry a variety of techniques are known for transferring designs to paper or films, for example, by means of print originals. One possibility is that known as flexographic printing.
In the flexographic printing process, flexible printing plates are bonded to printing cylinders or printing sleeves. Such plates consist, for example, of a polyethylene terephthalate film (PET film) on which there is an applied photopolymer layer into which the appropriate print relief can be introduced by exposure of the print elements and subsequent washing-out of the non-print elements. The plate is then bonded to the printing cylinder or printing sleeve by way of the PET film.
For the bonding, generally speaking, double-sided pressure-sensitive adhesive tapes are used, on which very stringent requirements are imposed. For the printing operation, the pressure-sensitive adhesive tape is required to have a certain hardness, but also a certain elasticity. These properties must be set very precisely in order that the printed image produced yields the desired outcome in accordance with the requirements. Stringent requirements are likewise imposed on the pressure-sensitive adhesive (PSA), since the bond strength ought likewise to be sufficient so that the printing plate does not detach from the double-sided pressure-sensitive adhesive tape, or the pressure-sensitive adhesive tape from the cylinder or sleeve. This must be so even at elevated temperatures of 40 to 60° C. and at relatively high printing speeds. In addition to this property, however, the PSA must also possess reversible adhesion properties, to allow the printing plates to be detached again after the printing operations (in that situation, the adhesive bond between the pressure-sensitive adhesive tape and the print cylinder or print sleeve, and also the bond to the plate, must be able to be parted without residue, in order to ensure that both components can be used again). This detachability ought also to exist after bonding over a relatively long period (up to 6 months). It is desirable, moreover, for it to be possible to remove the pressure-sensitive adhesive tape and especially the printing plate without destruction thereof, and also without great application of force, since in general the printing plates are used a number of times. Furthermore, there should be no residues on the printing plate and on the cylinder or sleeve. In summary, therefore, very exacting requirements are imposed on the double-sided pressure-sensitive adhesive tapes suitable for this utility.
Residue-free redetachability is a problem especially in the case of polar substrates such as steel, for example, since here it has been found that the bond strengths increase considerably over the course of time. For the purposes of the present specification, in relation to surfaces, the terms “polar” and “high-energy”, i.e., having a high surface energy (SE), are equated, as are the terms “nonpolar” and “low-energy”, since this simplifying model has become established in the art. The finding that lies behind this is that polar dipole forces are comparatively strong relative to what are called “disperse” or nonpolar interactions, which are built up without participation of permanent molecular dipoles. The basis for this model of interfacial energy and interfacial interactions is the idea that polar components interact only with polar components, and nonpolar components only with nonpolar components.
This energy and its components are often measured by measurement of the static contact angles of different test liquids. The surface tensions of these liquids are assigned polar and nonpolar components. From the contact angles observed between the droplets and the test surface, the polar and nonpolar components of the surface energy for the surface under test are ascertained. This can be done, for example, according to the OWKR model. One alternative method customary industrially is the determination using test inks according to DIN ISO 8296.
Examples of pressure-sensitive adhesives include those based on natural rubber, as documented by EP 760 389 A. Also employed for the stated utility, however, are pressure-sensitive adhesive tapes having polyacrylate-based PSAs. Accordingly, for example, WO 03/057497 A describes an acrylate PSA based on block copolymer for the stated application. WO 2004/067661 A discloses a pressure-sensitive adhesive tape with a PSA based on a soft acrylic monomer (TG<−20° C.) composed of at least 49.5 wt % of a hard, cyclic or linear (meth)acrylic ester monomer (TG≧30° C.) and at least 10 wt % of functionalized hard (meth)acrylic/ester monomers (TG≧30° C.), the PSA being produced in a two-stage method.
A further disadvantage of many PSAs known from the prior art for the adhesive bonding of printing plates is manifested especially when the bonded printing plates are to be cleaned to remove the printing ink. This is normally brought about by using the solvents, which also serve as solvents for the inks themselves, in large quantities for washing and removing the inks from the plates. Inevitably in this procedure, however, there is creepage below the edges of the bond of the plate on the pressure-sensitive adhesive tape, and the edges of the adhesive tape on the printing cylinder or printing sleeve. This entails detachment of the bond (of the plate to the adhesive tape and of the adhesive tape to the cylinder or sleeve), since the adhesives of the pressure-sensitive adhesive tape lose the necessary adhesion. The lifted edges produced as a result (“flags”) prevent further printing operations by smearing the printing ink, if there are not, indeed, mechanical problems with the flags in the printing apparatus and hence system outages. In practice, therefore, the bonds on printing plates mounted with prior-art adhesives have to be protected from the solvent by sealing of the respective edges with single-sided pressure-sensitive adhesive tapes or with liquid adhesives or hotmelt adhesives.
This additional sealing operation implies a significant extra expense, and the risk exists of damaging the expensive printing plates on demounting, particularly where liquid adhesives or hotmelt adhesives are used.
EP 2 226 372 A1 discloses an acrylate-based PSA for the bonding of printing plates to cylinders or sleeves that has a high acrylic acid fraction of between 8 and 15 wt %. Further monomers are linear and branched acrylic esters, and are in a defined ratio to one another. Using such an adhesive, the requirements in terms of edge lifting behavior and solvent resistance are met very well. PSAs with a high acrylic acid fraction, however, lead to strong peel increase on polar substrates, such as steel, which is commonly the material for printing cylinders. This problem also arises with the adhesive of EP 2 226 372 A1, particularly if it is used on the side of the adhesive tape facing the printing cylinder or printing sleeve. Demounting such adhesives from such substrates, therefore, entails problems; very high demounting forces arise, and the adhesive tape used may fracture, or residues remain on the substrate.
In the tailoring of pressure-sensitive adhesion properties to particular end uses, the composition of the polymer component has a substantial influence. In the prior art there are a series of applications known that disclose acrylate-based PSAs where selection may be made from the pool of the parent monomers composed, among others, of hard acrylic monomers, stearyl acrylate, N-alkyl-substituted amides, such as N-tert-butylacrylamide, and acrylic acid, as for instance in DE 10 2004 002 279 A1, in DE 103 10 722 A1, in 103 12 031 A1, or else in DE 10 2008 023 758 A1. While the three former specifications disclose PSAs for other areas of application, DE 10 2008 023 758 A1 is directed to a use comparable with that of the present specification.
None of the cited documents, however, discloses a PSA based on a monomer mixture which corresponds specifically to the composition of the monomers set out above, even less so in specifically designated proportions to one another. Advantages arising for a PSA of this kind—in relation to the composition of DE 10 2008 023 758 A1 as well—especially in relation to the bonding of printing plates to polar substrates, therefore, cannot be inferred from any of the stated specifications, not even in combination with one another.
It is an object of the present invention to specify a PSA which even under the influence of solvents, ensures effective and reliable bonding to material common in flexographic printing, such as to PET (polyethylene terephthalate) in particular, but which nevertheless is still redetachable even after a very long time and even from highly polar substrates—such as the surfaces of print cylinders made from steel or the surfaces of polar plastics of defined printing sleeves. The PSA ought preferably to be suitable in particular for the reliable bonding of printing plates; for an adhesive tape with the PSA, the stability of the adhesive tape assembly, particularly the reliable anchoring of the PSA on foam carriers—such as polyolefinic foams—is to be ensured.